Ischemic Tolerance: In Situ and Remote Pre- and Postconditioning

Introduction

“Thus, when Heaven is about to confer a great office on any man, it first exercises his mind with suffering … it exposes his body to hunger ... it stimulates his mind, hardens his nature, and supplies his incompetencies.” More than 2000 years ago, the Chinese philosopher Mencius believed that a person who experiences prior suffering is able to take on more responsibility throughout life. Friedrich Nietzsche, the German philosopher, restates this as “That which does not kill us makes us stronger.” In the 1880s, the German pharmacologist Hugo Schulz described that the growth of yeast could be stimulated by small doses of poisons. In 1943, the term “hormesis” was coined, indicating that a low dose of poison is stimulating, whereas a high dose is toxic, an observation in almost all biological systems . This dose-dependent effect of hormesis, when applied to stroke research, is similar to the concept of ischemic tolerance. Ischemic tolerance occurs when a brief sublethal ischemic insult is applied before a subsequent injurious insult. The term ischemic tolerance is often interchangeable with the term ischemic preconditioning (PreC) but, more specifically, ischemic PreC refers to the brief, subinjurious ischemia conducted before a subsequent injurious ischemic insult, whereas ischemic tolerance refers to the increased ability of the ischemic organ to resist injury after the brief ischemic event. Ischemic tolerance or PreC is protective against ischemia in many organs, including the heart and brain. In addition, ischemic tolerance or PreC can be mimicked by cross-tolerance, which is induced by heterogeneous stimuli or stressors other than ischemia . The stimuli for PreC can also be applied after reperfusion, which is defined as postconditioning (PostC). In stroke, ischemic conditioning (both PreC and PostC) is performed in situ in the brain; thus it is defined here as in situ ischemic conditioning. In contrast to in situ ischemic conditioning, remote ischemic conditioning refers to ischemia conducted in an organ other than the brain, which can also attenuate brain injury after stroke.

In Situ PreC and PostC

The first study addressing in situ ischemic PreC against brain injury in 1964 described a prior anoxic insult that was found to protect against hippocampal CA1 neuronal death . The concept of ischemic PreC was formally defined in 1986 by Murry et al. in a myocardial ischemia model in dog , and it was replicated in the brain by Kitagawa et al. in a forebrain ischemic model in gerbil. In situ ischemic PreC in brain research has two therapeutic time windows—rapid and delayed. Rapid PreC is induced within a few hours, whereas delayed PreC is induced from 24 h to a few days, before the subsequent injurious cerebral ischemia or stroke. Ischemic PreC performed between these two time windows does not generate protective effects against brain injury. The protective effects of PreC have been proven in both in vivo and in vitro models. In vivo models include forebrain ischemia, focal cerebral ischemia, and hypoxia/ischemia, and in vitro models include neuronal cultures and organotypic brain slice cultures.

More recently, in situ ischemic PostC has also been found to protect against brain injury after cerebral ischemia or stroke . As mentioned earlier, PreC is a transient subinjurious ischemia conducted before ischemia onset, whereas PostC is performed after postischemic reperfusion. Typically, ischemic PostC refers to the interruption of reperfusion by using a mechanical method, which causes brief and repeated occlusions of the cerebral blood vessels after reperfusion. Nevertheless, ischemic PostC can also be induced by a single, brief period of ischemia or anoxia, and in vitro models of hypoxic PostC have also been established by using the oxygen-glucose deprivation model . In contrast to ischemic PreC, three therapeutic time windows for ischemic PostC have been arbitrarily defined—rapid, intermediate, and delayed. Rapid PostC is conducted immediately or within a few minutes after reperfusion; intermediate PostC is performed from a few hours to 12 h after reperfusion; and delayed PostC can be induced from 24 h to a few days after reperfusion . Rapid and intermediate PostC robustly reduce infarct sizes as measured 2–3 days after stroke, and were found to offer long-term protection for up to 2 months. Like rapid PostC, delayed PostC that is conducted 2 days after forebrain ischemia can attenuate hippocampal neuronal death. Nevertheless, delayed hypoxic PostC performed on day 5 after stroke does not reduce infarction, but it can attenuate delayed thalamic atrophy .

Cross-tolerance Induced by Heterologous Conditioning

Past studies have revealed the extensive pathological mechanisms of brain injury after stroke. Brain ischemia results in ATP depletion, which leads to ion disruption and imbalance in the distribution of Na ⁺ /K ⁺ across cellular membranes. This redistribution of ions results in anoxic or ischemic depolarization, and leads to glutamate release and increased intracellular Ca ²⁺ , which then initiates various cascades in the cell signaling pathways of both apoptosis and necrosis. In general, any hazard stimuli that can induce one of these pathological cascades can be used as a stimulus for PreC. Accordingly, PreC induced by a stimulus or stressor other than ischemia is called heterologous conditioning, whereas ischemic PreC is defined as homologous PreC. The protective effects induced by heterologous conditioning are referred to as cross-tolerance.

In summary, in addition to ischemic homologous PreC, ischemic tolerance can be also induced by many stimuli, including anoxia/hypoxia, hyperoxia, glutamate/N-methyl-d-aspartate, cerebral spreading depression, anesthesia, hypothermia/hyperthermia, inflammation (lipopolysaccharide), free radicals, and metabolic inhibition. Like ischemic PreC, ischemic PostC can also be mimicked by a broad range of stimuli and stressors . Until recently, these types of conditioning were conducted in the brain as in situ conditioning. Now ischemic tolerance can also be induced remotely, as described in the following section.